Border Disease (BD) of sheep is a viral induced dysmyelinating disorder caused by a virus in the genus Pestivirus and the family Togaviridae. BD virus, when acquired congenitally, causes abortions or multiple malformations of the CNS, the skeletal and immune systems and of the integument. In the CNS, the affected lambs have cerebellar tremors, are microencephalic and the only histopathological lesions seen in this disease are a reduction in myelin and a glial proliferation. This research program focuses on identifying the mechanisms of microencephaly and myelin reduction in these lambs with congenital BD. Experimental methods involve pathogenesis studies using immunoglobulins as anatomical and biochemical probes of the nervous system as well as biochemical methods to chracterize and isolate BD viral polypeptides. In our previous studies we demonstrated that persistent BD virus could only be acquired by a congenital infection, suggesting a viral trophism for precurser cells in the CNS and the lymphoreticular system. To test this hypothesis, primary cell cultures have been developed from fetal and adult ovine CNS tissues and from peripheral white blood mononuclear cells. Control sheep and a lamb with congenital BD were studied. Using dual immunocytochemical labeling studies, we found that all cell types in tissue culture from fetal and adult ovine tissues were susceptible to BD viral infection. These findings suggest that the restriction of BD viral replication in the animal may not be due solely to cell trophism for precurser cells. In preparation for electron microscopic localization of BD viral proteins, we have developed the techniques to visualize the BD virion, which is spherical and 70 nm in diameter. We have also been able to prepare infected cells where cellular morphology and BD viral antigenicty are maintained. These data suggest that the BD viral proteins are intracellular and associated primarily with membranes. BD virus in sheep and in tissue culture is an important tool for investigating the mechanisms of myelinogenesis and viral persistence.